Source test cascade impactor

ABSTRACT

A cascade impactor for measuring the quantity and size distribution of suspended particles in stacks, ducts and other pollution sources is described. The impactor which is adapted to be placed within the duct carrying the particle-laden fluids comprises a tubular body containing a plurality of serially spaced-apart impactor plates interspaced between serially spacedapart jet stages, each succeeding jet stage having a smaller gas flow cross section than the preceding jet stage. A sized portion of the particulate matter suspended in gases flowing through the cascade impactor is captured by collection surfaces on the impactor plate placed below the specific jet stage which imparts a sufficient amount of inertia to the particle for it to impinge upon the collection surface. Dry collection surfaces are used for liquid aerosols whereas collection surfaces covered with a sticky substance such as grease are utilized for dry particulate matter. The size distribution of the particulate matter is reflected by the amounts of particulate matter adhering to the various impactor plates.

United States Patent Pilat 51 Sept. 26, 1972 [54] SOURCE TEST CASCADEIMPACTOR [72] Inventor: Michael J. Pllat, Seattle, Wash.

[73] Assignee: The Battelle Development Corporation, Columbus, Ohio [22]Filed: Feb. 24, 1971 [21] Appl. No.: 118,408

[52] U.S. Cl. ..73/432 PS, 73/28 [51] Int. Cl. ..G0ln 15/02 [58] Fieldof Search ..73/432 PS, 28; 209/139 R [56] References Cited UNITED STATESPATENTS 3,001,914 9/1961 Andersen ..73/28 X 3,127,763 4/1964 Lippmann..'.....73/28 FOREIGN PATENTS 0R APPLICATIONS 1,170,047 11/1969 GreatBritain ..73/28 128,199 1/1959 U.S.S.R. ..73/432 PS OTHER PUBLICATIONSMercer, T. T., A Cascade lmpactor Operating at Low Volumetric FlowRates, in Lovelace Foundation Report, p. 1- 19, Dec. 1962.

Primary Examiner-Louis R. Prince Assistant Examiner-Joseph W. RoskosAttorney-Christensen & Sanbom [57] ABSTRACT A cascade impactor formeasuring the quantity and size distribution of suspended particles instacks, ducts and other pollution sources is described. The impactorwhich is adapted to be placed within the duct carrying theparticle-laden fluids comprises a tubular body containing a plurality ofserially spaced-apart impactor plates interspaced between seriallyspaced-apart jet stages, each succeeding jet stage having a smaller gasflow cross section than the preceding jet stage. A sized portion of theparticulate matter suspended in gases flowing through the cascadeimpactor is captured by collection surfaces on the impactor plate placedbelow the specific jet stage which imparts a sufficient amount ofinertia to the particle for it to impinge upon the collection surface.Dry collection surfaces are used for liquid aerosols whereas collectionsurfaces covered with a sticky substance such as grease are utilized fordry particulate matter. The size distribution of the particulate matteris reflected by the amounts of particulate matter adhering to thevarious impactor plates.

4 Claims, 3 Drawing Figures Z 7 gir 2 PATENTEU 1972 3,693,457

sum 2 or 2 SOURCE TEST CASCADE IMPACTOR BACKGROUND OF THE INVENTION Thisinvention relates to a method for and means of I sampling theparticulate matter content of fluid suspensions and in particulargaseous systems carrying suspended particulate matter flowing within theconfines of a duct or the like.

This invention further relates to an impaction type of sampling deviceby which a particle size distribution may be determined for aparticulate-laden gaseous stream.

The invention described herein was made in the course of work under agrant or award from the Department of Health, Education and Welfare.

Information on the quantities and size distributions of aerosols andother particulates in ducts and stacks is of use for aerosol inhalationstudies, designing particulate collection equipment, characterizing theaerosol emissions from air pollutant sources and other purposes. Toobtain a useful sample of and information about a gaseous streamcontaining suspended particulates, it is necessary to use a samplingmethod which has substantially isokinetic sampling capabilities, andwhich minimizes wall losses and water vapor condensation. The methodmust provide a representative sample of the gases and the apparatus usedmust have structural ruggedness, low cost, and ease of use to enablewidespread usage in industry.

Numerous prior art devices are known which utilize a multiple or singlejet stage with a collection plate located downstream from the jet stage.Heretofore the impingement devices have suffered from severe problems oflosses of particles to the walls and other cations as well asinefficiencies of various types to the extent thatsubstantially-unusable information is obtained from the quantity ofparticles which impinge upon and are collected by the collection platesin such devices. The wall losses and other associated problems havelargely been due to the failure of the prior art devices to provide aproper gas flow path for the particle-laden gases after impingement uponthe individual impingement stages To prevent collection of particles onthe walls and other surfaces, the flow path must have a sufficientcross-sectional area so that the gas velocities do not cause theparticles to impinge upon the walls or other surfaces of the collector.The design of prior 'art devices have frequently resulted in collectionof one half or more of the particulate matter in areas other than theimpingement plate of the device. Data gathered from such devices cannotreflect the true particle size distribution.

Other problems which have been associated with the prior art devices andwhich aggravate the problem of collection of particles on surfaces otherthan the collection plates include the problems of condensation ofliquids and failure of such devices to obtain a truly representativesampling of the particle-laden gases.

OBJECTS OF THE INVENTION It is therefore an object of this invention toprovide a source test cascade impactor which removes all or nearly allof the particulate matter suspended in a gas and classifies theparticulate matter into a size distribution.

It is a further object of this invention to provide a cascade impactorwhich may be suspended within the duct carrying the gas being sampled sothat problems of condensation and thermal collection of particles areminimized or eliminated.

An additional object of this invention is to provide apparatus andmethod for removing a representative particle-laden gaseous sample froma gaseous media and separate the particulate matter suspended thereininto a particle size distribution employing a multiple stage cascadetype impactor which has a plurality of gas-accelerating jets in eachstage and which has an increasing gas velocity and decreasing gas flowcross section serially in each stage along the direction of flow andwhich has a decreasing absolute gas pressure from the impactor inlet tothe impactor outlet.

A further additional object of this invention is to provide a cascadeimpactor in which all or nearly all of the particles collected byimpaction are collected upon the impactor plates rather than upon theside walls or other areas of the device.

One important object of this invention is to provide a cascade impactorcapable of handling substantial pressure drops across the individualstages thereof without gas leakage around the stages and in which theabsolute gas pressure and gas flow at the final jet stage may bemonitored to permit accurate calculation of the dimensions of particlescollected on the various impactor stages.

One specific object of this invention is to provide a multiple stagecascade type impactor for evaluation of particulate matter suspended ina flowing gaseous stream which provides an adequate gas flowcross-sectional area for flow of particle-laden gases between theindividual stages to substantially eliminate impingement of theparticulate matter on areas of the apparatus other than those areasintended for particle collection.

Another specific object of this invention is to provide a cascadeimpactor having a plurality of particle collection surfaces and a filtermeans to collect all particles not removed by impingement upon theparticle collection surfaces.

SUMMARY OF THE INVENTION The source test cascade impactor of thisinvention comprises a substantially tubular body having a plurality ofjet impingement stages with particle collection plates positionedbeneath the jet impingement stages contained within the interior of thetubular body. The entire body is adapted to be inserted inside ductcarrying particulate-laden fluids to be sampled and has an exteriorshape which minimizes aerodynamic drag and turbulence within the duct.

The tubular body has removable inlet and outlet sections connected to afirst and second end of the body, respectively. The inlet sectioncarries a nozzle means which may be interchanged with other nozzle meansto provide the particular nozzle gas flow cross section necessary toachieve isokinetic sampling of the gases.

The tubular body contains a plurality of particle collection stageswhich each comprise a jet stage element having a plurality of aperturesfor gas flow positioned ahead of a corresponding impactor plate towardwhich the particle-laden gases are caused to flow from the apertures.Each of the jet stage elements has a sealing means positioned betweenthe outer periphery thereof and the interior wall of the tubular body.The impactor plates are constructed so that a gas flow cross sectionsubstantially larger than the total gas flow cross section of the jetstages is afforded to the gas flowing past the impactor plates. This isaccomplished by either having an unimpeded peripheral flow around theouter edges of the impactor plates or by having the central portion ofeach impactor plate removed for gas flow through the center thereof. Inthe latter embodiment, the impactor plates take a substantially annularshape and the jet stage has the apertures therein arranged in a mannerso that the gases passing therethrough will impinge upon the outer ringportion which forms the collection surface of the substantially annularimpactor plate.

The gas flow cross section provided through each jet stage seriallydiminishes along the direction of flow of the fluid so that each stageimparts a higher velocity to the particle-laden fluid passingtherethrough than was experienced in the immediately preceding stage. Ateach stage particles which attain a sufficient inertia to strike thecollection surface of the impactor plate are captured and the smaller,lighter particles pass on to the next stage.

After passage of a quantity of gases through the impactor, the impactoris removed from the gas flow stream, dismantled and the quantity ofparticulate matter collected upon each stage determined. Empirical ortheoretical analysis may then be utilized to determine the particulatesize distribution. The analysis includes the calculation at the 50percent particle collection efficiencies of each jet stage using theparticle weight per collection plate, the gas temperature, gas absolutepressure, the gas volumetric flow made through the impactor, the numberof jets per stage and the diameter of the jet stages.

These and other objects, advantages and attributes of this invention maybe more readily ascertained by an evaluation of the followingdescription of the preferred embodiments with reference to the attacheddrawings.

IN THE DRAWINGS:

FIG. 1 shows a schematic diagram of a typical gas sampling trainutilizing the invention disclosed herein.

FIG. 2 shows a cross-sectional view of one embodiment of the samplingdevice of this invention.

FIG. 3 shows a cross-sectional view of a second embodiment of thisinvention.

Referring more particularly to the drawings, there is seen in FIG. 1 atrain having the cascade type impactor suspended within a duct having agas flow as shown. The cascade impactor has a nozzle 12, a cylindricalbody 14 and a filter 16, all of which are suspended within the interiorof the duct 10. By controlling the flow rate of gases through theimpactor, isokinetic conditions can be obtained. In addition, thepresence of the impactor within the interior of the duct substantiallyeliminates the problems of condensation of vaporous materials containedin the gas stream and the thermal deposition of particles upon coldersurfaces of the cascade impactor, since the cascade impactor assumes thesame temperature as the gas flowing in the duct 10 after a reasonableexposure time. Tubing 18 attached to the back of filter 16 passes outthrough the duct opening 20 and to a typical gas train having impingers22 and 24 positioned if desired within a constant temperature bath 23and a dry gas meter 26 equipped with temperature and pressure gauges 30,as well as gas flow meter 31 which records the volume of gas flowingthrough the meter. The flow of gas is controlled by valve 32 at theentrance to vacuum pump 28.

Referring to FIG. 2, a cross-sectional view of a first embodiment of thecascade impactor of the invention is shown. Gas inlet 40 is connected toa nozzle (not shown) for withdrawing gas from a flow within a duct orother location. The tubular body 42 contains a series of particlecollection stages comprising a jet stage and an impactor plate arrangedwith the apertures of the jet stages positioned directly above acollection surface on the impactor plates. Gases passing through theapertures in the jet stages are accelerated to impart sufficientvelocity to particles above a certain size so that those particlesimpact upon and are captured by a collection surface on the impactorplate.

In FIG. 2, a first jet stage 44 is shown positioned above first impactorplate 45 so that large particles entrained in the gas will impact uponthe collection surface of impactor plate 45. Jet stage 44 is sealedalong the walls of the cylindrical housing 42 by means of an 0 ring seal54.

Gas flow within the embodiment shown in FIG. 2, as indicated by thedotted arrows and proceeds through the jet apertures 44a, impinges uponthe upper surface of the first impactor plate 45 and then flows past thejet spacers 56 and around the edges of impactor plate 45. The impactorplate 45 is supported by plate spacer 57 spaced around the periphery ofthe impactor plate 45. The jet spacers 56 and plate spacers 57 may bemade as an integral part of the jet stages, the impactor plates, or maybe in the form of a spider or individual spacer elements. The gasproceeds along the indicated flow path through to the second jet stage46 and through the apertures 46a thereof, impinging upon the uppersurface of impactor plate 47. The smaller diameter of the apertures 46a,as compared with apertures 44a, cause the gas flowing therethrough toachieve a higher velocity so that particles smaller than those impingedupon plate 45 will impinge upon plate 47 and be collected. The gas thenflows around the circumference of the impactor plate 47 through jetstage 48 to impactor plate 49 and so on through the apparatus downthrough the last jet stage 50 to impact upon the last impactor plate 51.The apertures in the last jet stage 50 are quite small and causerelatively tiny particles to impinge upon impactor plate 51. Anyremaining particulate matter passes around the outside of impactor plate51 and is collected by the filter 58 which is held in position by afilter support plate 55 and an annular collar 53. The gas flow thenpasses out of the cascade impactor and into a typical gas train as shownin FIG. 1. Absolute gas pressure may be measured at the entrance to thegas train to permit theoretical analysis of the cascade impactoroperating characteristics.

In FIG. 3, a second embodiment of this invention is shown. The tubularhousing 62 has a gas inlet at one end and a gas outlet 94 at the otherend. The gas inlet 60 draws particle-laden gases into the interior ofthe cascade impactor and causes the gas to impinge upon the top impactortray 64. This impactor tray is designed to remove any very largeparticulate matter from the gas stream and cause the gas stream to flowout around the periphery of the impactor tray 64 into apertures 65 whichact as gas flow straighteners or distributors. An impactor plate may beplaced directly beneath this top impactor tray 64; however, for propergas flow, it is usually desirable to utilize an open space with a firstjet stage 66 positioned beneath the impactor tray 64. Apertures 67 inthe first jet stage 66 imparts sufficient velocity to the gases that thelarger particulate matter suspended therein impacts upon the collectionsurface of the first impactor plate 68 and is captured thereon. The gasthen flows toward the center of the impactor plate 68 and downwardlythrough the gas flow opening 69 into the next chamber where itencounters the second jet stage 70. The apertures 71 in the second jetstage 70 cause the gas to attain a sufficient velocity that medium sizedparticles are then impinged upon the collection surface of the secondimpactor plate 72. Gas flow then proceeds into the center of the secondimpactor plate 72 and flows downwardly through the gas flow opening 73.Further downward flow of the gases results in the same series of eventsas described above with the exception that each succeeding jet stage hassmaller apertures so that a higher velocity is imparted to the gasesflowing through the apertures, causing impingement of successivelysmaller and smaller particles upon the collection surface of eachsucceeding impactor plate. The gases ultimately reach the last jet stage74,

pass through its apertures 75 and impinge upon the last impactor plate76, collecting a portion of the smallest particles remaining in thegases. The gases then travel through gas flow opening 78 and throughfilter 80 which removes the very small particles remaining in thegaseous stream. Filter 80 is supported by the perforated disc 85 andheld in place by spacer 86.

Each jet stage is sealed from the adjacent stage by means of an 0 ringseal 82 and the upright portion 90 on each impactor plate provides thespatial separation necessary for proper operation of the cascadeimpactor. An annular spacer 92 is positioned between the bottom of eachimpactor plate and the top of each jet stage to provide the necessaryspacing therebetween. The annular spacers may be made as an integralpart of the jet stages or the impactor plates, or may be individualelements positioned in the impactor to provide the necessary spacing.The gas outlet 94 is connected to a vacuum source such as a typical gastrain for sampling gases.

Use of the invention in either of its embodiments may be carried out bythe following series of preparatory and sampling steps. The cascadeimpactor is thoroughly cleaned by washing with a suitable solvent suchas benzene to remove any materials remaining from previous testing. Thecollection surface of each impactor plate is then coated with a thinlayer of a tacky or sticky substance such as grease in the event thatsolid particulate matter is being sampled. If the particulate matterbeing sampled comprises liquids or liquid aerosols, no grease will benecessary on the surface of the impactor plates. The impactor plates andthe filter in the bottom of the cascade impactor are then individuallyweighed accurately to determine a tare weight.

The conditions of the gas flowing in the duct is then determined byevaluating the gas velocity profile and measuring the temperature,pressure, humidity and other conditions. Based upon these conditions, anozzle size is calculated which will provide isokinetic sampling.

It is also usually necessary to find a suitable location for thesampling to take place in which the gas flow is reasonably constant andpreferably in the laminar flow regime. It is preferable to have thesampling location approximately 20 diameters length down the duct from adiscontinuity to permit the gas flow in the duct to attain even, uniformconditions if possible. The sampling train composed of the cascadeimpactor followed by suitable filtration or impinging equipment and agas dry meter connected to an air pump is set up in preparation forsampling. Water condensation problems may be prevented by preheating thecascade impactor for a sufficient amount of time to permit the impactorto assume a temperature substantially equal to that present on theinside of the duct, or the impactor may be placed directly into the ductand held there long enough to achieve thermal equilibrium beforebeginning sampling.

The cascade impactor is then placed at the desired location within theduct and a substantially constant gas flow rate established through theimpactor. The apparatus is operated for a period of time sufficient togive a suitable sample size for determination of the particle quantityand size distribution within the duct.

The cascade impactor is then removed from the duct and dismantled forevaluation of the captured particles on each impactor plate. The platesare weighed to determine the quality of particulate matter collected byeach plate and the filter resulting in an indication of the sizedistribution of the particles entrained in the gases contained in theduct. The size distribution of particles may be determinedmicroscopically to calibrate the cascade impactor or the theoreticalcalibration equation suitable for use with this type of apparatus may beutilized. Chemical analysis of the particle size fractions may also beperformed.

In one example of the application shown in H6. 2 which was constructedand tested for its collection efficiency, the following dimensions wereutilized. A cylindrical aluminum tube 3 inches in diameter formed thetubular body housing the impactor stages. Each jet stage had a thicknessof 0.05 inches so that the axial length of apertures throughout thedevice were 0.05 inches. The distance separating the bottom surface ofthe jet stage and the top surface of the impactor plates was 0.1 inch.The number of jets and the jet diameter for each stage is listed belowin Table 1.

The cascade impactor described above was used to collect fly ash inboiler stacks, measure the particulate size distribution in the fluefrom a kraft pulp mill recovery furnace and other test situations. Someproblems with loss of a minor amount of the particulate matter to thewalls and to the top surface of the first jet stage were encountered.The particle size distribution of suspended particulate matter waseasily ascertained by weighing the amount of particulate mattercollected on each stage and applying well known theoretical analysis tothe impingement collection efficiencies for each plate.

Particle collection observed with models of the cascade impactordescribed above and that observed by sampling with an Alundum thimblefilter were substantially the same if particles on top of the firststage and on the walls were included in the total particles collected.

The embodiment of this invention shown in FIG. 3 substantiallyeliminates all the wall loss problem and by its construction preventsthe accumulation of particulate matter on top of the first jet stage.The centrally located fluid passageway through each impingement platecauses the particle-laden fluid to flow to the center of the device,thus avoiding any contact with the walls.

In considering the invention, it will be understood that the inventionis not limited to the particular embodiments described in detail abovenor is the cascade impactor of this invention limited to any particularnumber of stages, to the materials of construction utilized nor tospecific structural details. While the cylindrical form for the tubularbody is preferred, any suitable shape may be provided which permits easyassembly and disassembly of the cascade impactor. Therefore, in thepractice of the invention, numerous changes in the structure materialsand configuration may be made without departure from the spirit or scopeof the invention as defined in the appended claims.

lclaim:

l. A cascade impactor for sampling particle-laden gases for totalparticulate content and particle size distribution adapted to be placedwithin the fluid stream being sampled comprising:

a tubular body having a gas inlet at one end and a gas outlet at theother end, said gas inlet including a nozzle means sized for isokineticsampling;

a plurality of particle collection stages positioned within said tubularbody, said stages each including a jet stage having a plurality ofapertures therethrough cooperating with an impactor plate having aparticle collection surface thereon, said surface positioned adjacentthe outlet of said apertures, the total cross-sectional area of saidapertures in each jet stage diminishing serially in the direction offlow of said fluid;

a fluid flow path between adjacent particle collection stages permittingrelatively unrestrained flow of fluid from the impactor plate of one ofsaid particle collection stages to the upper surface of the immediatelysubjacent jet stage, said flow path past said impactor stages being acentrally located aperture in each of said plates, and spacer meansmaintaining said et stages and said impactor plates in a spaced apartrelationship along the axis of said tubular member.

2. The apparatus of claim 1 wherein said impactor plates each comprisean annular surface having the open center thereof comprising said fluidflow path and said jet stages each having said plurality of aperturesarranged upon a substantially annular portion thereof so thatparticle-laden fluids pass through said apertures and impinge upon saidannular surface thence through said open center to the upper surface ofthe immediately subjacent jet stage.

3. The apparatus of claim 1 wherein said body is substantiallycylindrical.

4. A cascade impactor for determining the size distribution of particlesand total particle content of particle-laden gases comprising:

a tubular body having a gas inlet at a first end and a gas outlet at asecond end thereof, and

a plurality of particle collection stages placed within said tubularbody, said collection stages each comprising a jet stage peripherallyengaging the interior wall of said tubular body in a fluid sealedrelationship and having a plurality of equally sized aperturestherethrough and an impactor plate having a particle collection surfacespaced from and positioned substantially parallel to the adjacent jetstage, at least a portion of the outer edge of said impactor plate beingspaced from the interior wall of said tubular body to define aperipheral flow opening for gases past said impactor plate, theperipheral region of said particle collection surface beingsubstantially flat without projections extending toward said inletthereby substantially preventing impaction of particulate matter uponsurfaces other than said particle collection surface, said collectionstages having fluid flow cross-sectional areas in the jet stage thereofserially diminishing in the direction of flow of said particle-ladengases.

1. A cascade impactor for sampling particle-laden gases for totalparticulate content and particle size distribution adapted to be placedwithin the fluid stream being sampled comprising: a tubular body havinga gas inlet at one end and a gas outlet at the other end, said gas inletincluding a nozzle means sized for isokinetic sampling; a plurality ofparticle collection stages positioned within said tubular body, saidstages each including a jet stage havIng a plurality of aperturestherethrough cooperating with an impactor plate having a particlecollection surface thereon, said surface positioned adjacent the outletof said apertures, the total cross-sectional area of said apertures ineach jet stage diminishing serially in the direction of flow of saidfluid; a fluid flow path between adjacent particle collection stagespermitting relatively unrestrained flow of fluid from the impactor plateof one of said particle collection stages to the upper surface of theimmediately subjacent jet stage, said flow path past said impactorstages being a centrally located aperture in each of said plates, andspacer means maintaining said jet stages and said impactor plates in aspaced apart relationship along the axis of said tubular member.
 2. Theapparatus of claim 1 wherein said impactor plates each comprise anannular surface having the open center thereof comprising said fluidflow path and said jet stages each having said plurality of aperturesarranged upon a substantially annular portion thereof so thatparticle-laden fluids pass through said apertures and impinge upon saidannular surface thence through said open center to the upper surface ofthe immediately subjacent jet stage.
 3. The apparatus of claim 1 whereinsaid body is substantially cylindrical.
 4. A cascade impactor fordetermining the size distribution of particles and total particlecontent of particle-laden gases comprising: a tubular body having a gasinlet at a first end and a gas outlet at a second end thereof, and aplurality of particle collection stages placed within said tubular body,said collection stages each comprising a jet stage peripherally engagingthe interior wall of said tubular body in a fluid sealed relationshipand having a plurality of equally sized apertures therethrough and animpactor plate having a particle collection surface spaced from andpositioned substantially parallel to the adjacent jet stage, at least aportion of the outer edge of said impactor plate being spaced from theinterior wall of said tubular body to define a peripheral flow openingfor gases past said impactor plate, the peripheral region of saidparticle collection surface being substantially flat without projectionsextending toward said inlet thereby substantially preventing impactionof particulate matter upon surfaces other than said particle collectionsurface, said collection stages having fluid flow cross-sectional areasin the jet stage thereof serially diminishing in the direction of flowof said particle-laden gases.